1. Field of the Invention
The invention relates to communications, and more particularly to spread spectrum digital communications and related systems and methods.
2. Background
Spread spectrum wireless communications utilize a radio frequency bandwidth greater than the minimum bandwidth required for the transmitted data rate, but many users may simultaneously occupy the bandwidth. Each of the users has a pseudo-random code for “spreading” information to encode it and for “despreading” (by correlation) the spread spectrum signal for recovery of the corresponding information. FIG. 2 shows a system block diagram, and FIGS. 3a-3b illustrates pseudo-random code plus a QPSK (quadrature phase-shift keying) encoder. This multiple access is typically called code division multiple access (CDMA). The pseudo-random code may be an orthogonal (Walsh) code, a pseudo-noise (PN) code, a Gold code, or combinations (modulo-2 additions) of such codes. After despreading the received signal at the correct time instant, the user recovers the corresponding information while the remaining interfering signals appear noise-like. For example, the interim standard IS-95 for such CDMA communications employs channels of 1.25 MHz bandwidth and a code pulse interval (chip) TC of 0.8138 microsecond with a transmitted symbol (bit) lasting 64 chips. The recent wideband CDMA (WCDMA) proposal employs a 3.84 MHz bandwidth and the CDMA code length applied to each information symbol may vary from 4 chips to 256 chips. The CDMA code for each user is typically produced as the modulo-2 addition of a Walsh code with a pseudo-random code (two pseudo-random codes for QPSK modulation) to improve the noise-like nature of the resulting signal. A cellular system as illustrated in FIG. 4 could employ IS-95 or WCDMA for the air interface between the base station and the mobile user station.
A spread spectrum receiver synchronizes with the transmitter by code acquisition followed by code tracking. Code acquisition performs an initial search to bring the phase of the receiver's local code generator to within typically a half chip of the transmitter's, and code tracking maintains fine alignment of chip boundaries of the incoming and locally generated codes. Conventional code tracking utilizes a delay-lock loop (DLL) or a tau-dither loop (TDL), both of which are based on the well-known early-late gate principle.
In a multipath situation a RAKE receiver has individual demodulators (fingers) tracking separate paths and combines the results to improve signal-to-noise ratio (SNR), typically according to a method such as maximal ratio combining (MRC) in which the individual detected signals are synchronized and weighted according to their signal strengths. A RAKE receiver usually has a DLL or TDL code tracking loop for each finger together with control circuitry for assigning tracking units to received signal paths. FIG. 5 illustrates a receiver with N fingres.
The UMTS (universal mobile telecommunications system) approach UTRA (UMTS terrestrial radio access) provides a spread spectrum cellular air interface with both FDD (frequency division duplex) and TDD (time division duplex) modes of operation. UTRA currently employs 10 ms duration frames partitioned into 15 time slots with each time slot consisting of 2560 chips. In FDD mode the base station and the mobile user transmit on different frequencies, whereas in TDD mode a time slot may be allocated to transmissions by either the base station (downlink) or a mobile user (uplink). In addition, TDD systems are differentiated from the FDD systems by the presence of interference cancellation at the receiver. The spreading gain for TDD systems is small (8-16), and the absence of the long spreading code implies that the multi-user multipath interference does not look Gaussian and needs to be canceled at the receiver.
In currently proposed UTRA a mobile user performs an initial cell search when first turned on or entering a new cell; this search detects transmissions of base stations on the physical synchronization channel (PSCH) without any scrambling. The initial cell search by a mobile user must determine timing (time slot and frame) plus identify pertinent parameters of the found cell such as scrambling code(s).
For FDD mode the physical synchronization channel appears in each of the 16 time slots of a frame and occupies 256 chips out of the 2560 chips of the time slot. Thus a base station transmitting in the synchronization channel a repeated primary synchronization code of pseudo-noise of length 256 chips modulated by a length 16 comma-free code (CFC) allows a moblie user to synchronize by first synchronizing to the 256-chip pseudo-random code to set slot timing and then using the cyclic shift uniqueness of a CFC to set frame timing. Further, decoding the CFC by the mobile user reveals the scrambling code used by the base station.
In contrast, for TDD mode the physical synchronization channel only appears in one or two time slots per frame, so the length-16-CFC-modulated primary synchronization code does not easily apply. An alternative proposed TDD mode initial cell search employs a sum of a primary synchronization code (PSC) plus six secondary synchronization codes (SSCs); each code is a 256-chip pseudo-noise sequence, and the codes are orthogonal. In this proposal the initial cell search consists of slot synchronization, frame synchronization and code group identification with scrambling code determination. In particular, during slot synchronization the mobile user employs the PSC to acquire slot synchronization to the strongest cell (strongest received base station transmission): the PSC is common to all cells. A single matched filter (or any similar device matched to the PSC) may be used for detection.
Next, the mobile user employs the six SSCs to find frame synchronization and to identify one out of 32 code groups being used by the base station. Each of the six SSCs is modulated by +1 or −1; this implies 5 bits of information to identify which one of 32 possible code groups is used by the found base station (scrambling codes and midambles), and the sixth SSC is modulated by +1 or −1 to identify whether the time slot is the first or second physical synchronization channel slot in the frame (frame synchronization). Each of the six SSCs is scaled by 1/√6 to make the power of the sum of the six modulated SSCs equal to the power of the PSC. Lastly, the mobile user determines which of the four scrambling codes in the cell's code group is being used by, for example, correlation on the common control physical channel.
However, this TDD mode proposal has problems including low signal to noise ratio in the sum of the six modulated SSCs.
There is also a problem of efficient decoding within TDD mode proposals.